In a frequency division duplex (FDD) wireless transceiver, such as CDMA and WCDMA, the transmitter and receiver sections of a mobile station need to operate simultaneously. A typical RF FDD front-end single-band block diagram of the wireless transceiver is shown for example in FIG. 1.
The duplexer in the RF front-end is used to separate the transmission and reception signals. The duplexer specification requirements on suppressing unwanted signal and/or interference are very high. Typically 55 dB or greater of isolation is required to suppress the transmission signal leaking into the receiver and a minimum 45 dB is required to suppress the transmitter noise in the receiver frequency band. Excessive transmission leakage through the duplexer to the receiver will cause inter-modulation and/or cross-modulation interference desensitizing the receiver. An external SAW filter with modest rejection level (typically 20 dB) is often placed after the LNA to relax the mixer linearity and duplexer rejection requirements. However SAW filters historically have shown resistance to integration and frequency tunability, thus increasing the size, component count, and cost of the overall transceiver.
In order to address these problems, tunable solutions such as YIG filters have been proposed. These filters exhibit low loss and broad tuning bandwidth characteristics, but they require an externally applied magneto-static field, suffer from slow tuning times due to hysteresis effects, and exhibit high power consumption.
Other options include distributed filter designs using coupled sections of resonant printed structures such as loaded combine filters, loaded loop resonators, or interdigitated filters. However, the large footprint required by these designs becomes the main disadvantage for any distributed implementation when designed for operation at typical cell phone frequency bands (700 MHz-2.7 GHz). In general, either acoustic or tunable lumped element filters must be used to meet cell phone real estate constraints. A non-tunable notch filter using bond wire inductors operating at the IMT band (TX: 1.92-1.98 GHz, RX: 2.11-2.17 GHz) has been developed, but low suppression level (approx. 12 dB) and high insertion losses (3 dB) can be measured with such a system. In addition, due to the non-tunable nature of the design, the suppression level shows high variation within in the operating frequency band.
In the cellular systems, the mobile phone operation frequency is channelized. The channel frequency spacing is only 100 kHz to 200 kHz depending on systems. Thus, the narrow band notch filter frequency needs to have frequency accuracy within 100 kHz for 2.5G and 3G mobile stations (channel spacing is 200 kHz) or 50 kHz for the LTE mobile stations (channel spacing 100 KHz). The frequency accuracy of the notch filter in this kind of application needs to be about 10−5 at least if the effective bandwidth of the filter is narrow down to the extent close to the signal bandwidth. The accuracy of the components comprising the notch filter is only 1%. In this case, it is impractical for an individual notch filter to utilize a fixed lookup table for filter tracking the transceiver operation frequencies. Accordingly, it would be desirable for a tunable filter to be able to dynamically track transceiver operation.